There is very little selection - you are stuck with choosing between 2 models

They are non-repairable - most shops won't touch them for liability reasons

They ride very rough over sharp small bumps

They are rare - if you get a flat anywhere other than a major urban center, you are SOL - you will very likely have to buy a non-runflat (I have personal experience with this one)

The only thing I like about them is that the stiff sidewalls make for great turn-in feel, and they also hold up better at the track than any other street tire I've tracked.

Runflat Story
Last year on the way back from the Tail of the Dragon, a friend of mine in a runflat-equipped 135i hit a rock on the interstate in middle-of-nowhere Ohio. The stiff sidewall combined with a rock impact blew a hole the size of a quarter in the sidewall. I do not think a normal tire would have failed in this case - it would have taken the hit without blowing out the sidewall. We made it to the next exit and had to call a flatbed, which towed the car to Kelly BMW in Columbus, OH. We had to book a hotel since it was night. The next day neither the BMW dealer nor any of the local tire shops had the runflat we needed so he ended up buying a pair of Hankook V12s in order to have matching tires on the fronts. The pair of Hankooks cost less than the single runflat at the dealer, who also wanted us to wait half a day for the tire to be shipped in from another dealer.

Now if this was a pre-runflat BMW with a proper full sized spare in the trunk, we would have pulled off to the side of the interstate, installed the spare, and drove home to Toronto that night. No hotel room x 4 (there were 4 of us on the trip), no waiting 2 hours for a tow, no need to buy 2 tires of the wrong type, etc...